A high speed turbo machine, such as, for example, a steam or gas turbine, generally comprises a plurality of blades arranged in axially oriented rows, the rows of blades being rotated in response to the force of a high pressure fluid flowing axially through the machine. It is common to monitor the position of the blades relative to a flowpath wall within the turbine, both during the design and testing of the turbine and during normal operation of the turbine. For example, it is known to use non-contacting proximity sensors or probes to detect a gap distance between the blade tips and the flowpath wall, as well as detect blade vibrations.
In addition, control of blade-tip clearance in the compressor and turbine sections of gas turbine engines can improve efficiency, minimize leakage flow, and shorten engine development time. Tip clearance varies throughout different operating conditions (e.g., start-up, idle, full power, shut-down) because of different radial forces and different thermal expansion coefficients and heat transfer. A real-time clearance control system can lead to turbine designs that eliminate rubbing of the housing and minimize leakage flow for maximum engine efficiency. In particular, in a turbine design that features hydraulic clearance optimization (HCO), measurement of blade tip clearance can be especially beneficial.
One conventional proximity sensor includes a capacitance gap sensor that has a single sensing electrode that is energized by a voltage so as to generate an electric field in the expected path of a turbine blade. The sensor is located within a cavity of the turbine casing near where a blade will pass. The blade and casing of the turbine provide a virtual ground for the electrode such that the electrode and the blade act as a capacitor. When a turbine blade passes through the generated electric field, the capacitance between the electrode and the blade changes. A magnitude of the change in the capacitance between the electrode and the virtual ground is used as an indicator of a proximity of the turbine blade to the electrode.
The above approach has a number of drawbacks. In particular, the ambient conditions where the sensor is located affects the magnitude of a resulting change in the sensor's capacitance. Furthermore, the conditions within a turbine, such as near the first and second row, may reach temperatures of about 2500 C or more. Operation in such an environment can degrade the performance of a conventional capacitance gap sensor such that it may fall out of calibration in a matter of days or weeks. This is especially the case in gas turbine applications where it is critical to measure blade clearances in the turbine during the whole operation cycle. The sensor should be capable of working in environments including ambient air at atmospheric pressure during engine start up, in vitiated air that is the exhaust gas from the combustor with pressures in the 20-30 bar range and temperatures in the range of 1200 C to 1500 C at base load operation, and in hot air at quickly varying pressure and temperature during engine shut down.
Accordingly, there is currently an unmet need for a proximity sensor, for example a turbine blade proximity sensor, which provides accurate results in a variety of environments, over a relatively long period of time without re-calibration.